Gadolinium

Why are most MR contrast agents based on the element gadolinium?  

Complexes of the element gadolinium (Gd) are the most widely used of all MR contrast agents. Because of its unique electronic structure (described below), Gd is strongly paramagnetic. Paramagnetism is an intrinsic property of certain materials to become temporarily magnetized when placed in an external magnetic field. In fact, Gd is one of only four elements that can be magnetized at room temperature (the other three being iron, nickel, and cobalt).

The powerful paramagnetic properties of Gd make it extremely useful as an MR contrast agent. Gadolinium is not directly seen in an MR image, but manifests its presence indirectly by facilitating the relaxation of nearby hydrogen protons. Gd preferentially shortens T1 values in tissues where it accumulates rendering them bright on T1-weighted images.

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Paramagnetism may exist over a wide dimensional range — from subatomic particles to atoms to entire molecules. Nuclear paramagnetism, the form responsible for the NMR phenomenon, is extremely weak except in the immediate vicinity of the nucleus. It plays little or no role in determining the gross paramagnetic properties of entire atoms like gadolinium.

The form of paramagnetism exhibited by gadolinium compounds derives from electrons, not protons, and is known as Curie paramagnetism. Because electrons have the same spin (½) but a much smaller size than protons, their gyromagnetic ratios are 657 times larger. If these electrons remain unpaired in shells or bonding orbitals, the unbalanced spins produce a strong magnetic moment capable of inducing magnetic relaxation in nearby nuclei. This is the origin of the bulk paramagnetism possessed by elements such as gadolinium. 

Gadolinium has atomic number 64 on the periodic table. It occupies the central position in the lanthanide series of elements. Lanthanides are rare-earth metals grouped chemically because they possess partially filled inner shells of electrons (4f and 5d subshells). 

The electronic structure of the neutral Gd atom is shown right. Note the 7 unpaired electrons in its 4f subshell that account for the element's strong paramagnetism. In its ionized state, Gd⁺³ donates its 6s² and 5d¹ electrons for bonding, leaving its 4f⁷ electron shell intact. The powerful magnetic moment of Gd is therefore largely maintained even when chelated to a ligand such as DTPA in a contrast agent formulation.

Electron structure of the Gd atom. Note 7 unpaired electrons in its 4f shell, the most of any element.

 

Advanced Discussion (show/hide)»

There is some controversy whether Gd should be described as truly "ferromagnetic" at room temperatures, but it can indeed be magnetized. (See article by Coey et al below). Its transition temperature (Curie point) is about 19°C (66°F).

In addition to the powerful magnetic moment created by its 7 unpaired electrons, the Gd⁺³ ion also has a relatively long electron-spin relaxation time (τ_s ≈ 10⁻⁹ sec) at clinically relevant field strengths. This results from the symmetry of the Gd(III) S-state, creating a more hospitable environment for T1-relaxation than other lanthanide ions.

Other transition metal ions commonly considered as MR contrast agents include ₂₄Cr⁺³, ₂₅Mn⁺², ₂₆Fe⁺³, and ₂₉Cu⁺². These ions have 3, 5, 5, and 1 unpaired electron in their 3d shells. The two strongest of these (₂₅Mn⁺² and ₂₆Fe⁺³) have magnetic moments of approximately 5.9 Bohr magnetons, compared to ₆₄Gd⁺³ at 7.9.

Two other lanthanide ions, ₆₃Eu⁺³ and ₆₆Dy⁺³, have 6 and 5 unpaired electrons in their 4f shells, but again come up short compared to ₆₄Gd⁺³. Due to asymmetry of their electronic S-states, these lanthanides have electron spin relaxation times that are too short for effective T1 relaxation. They are used experimentally as T2 or T2* relaxation contrast agents.

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References
      Coey JMD, Skumryev V, Gallagher K. Is gadolinium really ferromagnetic? Nature 1999; 401:35-36.       "Gadolinium".  Wikipedia, the Free Encyclopedia.     
      Lauffer RB: Paramagnetic metal complexes as water proton relaxation agents for NMR imaging: theory and design. Chem Rev 1987; 87:901-927. 
     Moeller T. Periodicity and the lanthanides and actinides. J Chem Educ 1970; 47:417-423.
     Weinmann H-J, Brasch RC, Press W-R, Wesbey GE. Characteristics of Gadolinium-DTPA complex: a potential NMR contrast agent. AJR Am J Roengenol 1984; 142:619-624.

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Related Questions
     What are the causes of T1 and T2 relaxation?
     What is magnetic susceptibility?
     How does gadolinium cause relaxation?  

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